I'm not altogether happy with the post. I was torn between explaining more about the mathematics and moving on to the philosophical/methodological point, and I'm afraid I fell between two stools.
As for the basic idea behind modular arithmetic: we actually use it in certain everyday situations, in particular in connection with time. If it's 8 o'clock now, what time will it be in 6 hours? Not 14 (=8+6), but 2 (the remainder of 14 after dividing by 12). Clock-time, in other words, operates (more or less) modulo 12. (Military time is modulo 24.) Or think of an odometer, as it reaches its limit and rolls back to all zeros.
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As for the basic idea behind modular arithmetic: we actually use it in certain everyday situations, in particular in connection with time. If it's 8 o'clock now, what time will it be in 6 hours? Not 14 (=8+6), but 2 (the remainder of 14 after dividing by 12). Clock-time, in other words, operates (more or less) modulo 12. (Military time is modulo 24.) Or think of an odometer, as it reaches its limit and rolls back to all zeros.